4. Pretreatment of Sludge

Pretreatment technologies of sludge – thickening, conditioning, dewatering, drying – are discussed. Thickening is a fundamental stage of sludge pretreatment. Pre-dewatering technologies include rotary sludge thickeners, gravity thickeners, dissolved air floatation clarifiers and belt presses. The most widespread thickening method is gravity thickening. Sludge from wastewater treatment plants is frequently conditioned, using chemical or physical means to alter the floc structures of the sludge imparting sufficient stiffness and incompressibility to the structures so that water entrained in the sludge can rapidly be drained through filtering or other means. Dewaterability of the sludge is very important because it determines the volume of waste that has to be handled. Dewatering of sludge is performed by using vacuum filters, belt filter presses, centrifuges, and membrane filter presses. Centrifuges and belt filter presses are currently the most popular dewatering methods due to their good operation and cost efficiency. The primary sludges can be dewatered easily as these are high in fiber and low in ash. The most difficult are the solids from the high-rate biological treatment systems. The primary sludge most difficult to dewater is that containing ground wood fines. Drying of sludge using flue gases from combustion process is a standard method. Fluidized bed dryers, rotary dryers, and multiple hearth dryers are found to be effective for sludge drying.